A method of said type is known from DE 101 95 715 T5. According to the known method first an x-ray pulse in the low-energy range and then a further x-ray pulse in the high-energy range are emitted by the x-ray source. The radiation emitted by the x-ray source in the high-energy range and in the low-energy range penetrates an object under examination and is recorded by an x-ray detector, which in turn produces projection images of the object under examination. From the projection images recorded in the high-energy and low-energy range a combined image can then be produced by an evaluation unit connected downstream of the x-ray detector.
Because the absorption behavior of the irradiated material of the object under examination differs depending on the energy of the irradiating x-radiation, combined images can be produced by combining the projection images, said combined images reproducing the structural distribution of a specific material within the object under examination. For example structural distributions of two different materials having different absorption characteristics can be resolved when two projection images are recorded in different energy ranges.
A fundamental problem with such methods is that the temporal interval between the radiation pulse in the low-energy range and the radiation pulse in the high-energy range must not be allowed to become too long, because motion artifacts will otherwise occur in the combined image.
A further problem concerns semiconductor x-ray detectors, which must always be operated in a particular mode. The mode is defined by the number of detector elements read out, the read-out frequency and the duration of the x-ray window. X-ray window means the period of time during which the semiconductor x-ray detector can record x-radiation. A change of the mode in which the semiconductor x-ray detector is operated frequently leads to switching artifacts, which are also known as modeswitch artifacts. Current offset images are also necessary for every mode of the x-ray detector, in order to be able to perform offset adjustments to the recorded projection images. As the number of modes in which the detector is operated increases, the number of offset images required for the offset adjustment also increases. Thus the effort required for the offset adjustment becomes greater.
Since motion artifacts are furthermore to be expected in medical procedures, in the known method the offset images are recorded in a temporal interval from the projection images. Switching modes between recording in the high-energy range and recording in the low-energy range is also not possible. The duration of the x-ray window for recording in the high-energy range and recording in the low-energy range is thus equally long.
However the duration of the radiation pulse in the high-energy range is set to be smaller than the duration of the radiation pulse in the low-energy range, because the effective cross-section of the x-ray quanta in terms of material decreases as the energy of the x-ray quanta increases. With constant exposure time the x-ray detector receives a higher detector dosage from the radiation pulse in the high-energy range than from the radiation pulse in the low-energy range. For this reason in the known method the exposure time for the radiation pulse in the high-energy range is set lower than the exposure time for the radiation pulse in the low-energy range.
In order to obtain an adequate detector dosage during the radiation pulse in the low-energy range the tube current must be set high, since the exposure time must not be allowed to become so long as to unnecessarily increase the danger of motion artifacts.
In order to reduce the tube current in the transition from the x-ray source settings for the radiation pulse in the high-energy range to the settings for the radiation pulse in the low-energy range the incandescent filament of the cathode of the x-ray source must be cooled down. Since time is required for this, the radiation pulse in the high-energy range cannot immediately follow the radiation pulse in the low-energy range. Time is also required for reading out data from the x-ray detector. The x-ray detector readout takes place after the x-ray window has closed, and thus for this reason too it is not possible for the radiation pulse in the high-energy range to follow the radiation pulse in the low-energy range immediately.
Owing to this delay further motion artifacts can occur.